Flexible downhole tubing or “coiled tubing” is often extended downhole within a well to help facilitate various wellbore operations. Coiled tubing can reach upwards of 25,000 feet in length and is typically wound onto a large spool or reel from which it is drawn and progressively fed into the wellbore. Downhole tools can be coupled to the end of coiled tubing and simultaneously introduced into the wellbore to perform the various downhole operations. Since it is desirable to communicate with the downhole tools while in operation, communication lines and/or cables are often deployed with coiled tubing and communicably coupled to the downhole tools to facilitate communication between such downhole tools and the surface.
To deploy the cable with coiled tubing, the cable is often inserted or injected into the interior of the coiled tubing. In the past, to inject cable within coiled tubing, it was necessary to deploy the coiled tubing in a relatively straight line, such as deploying it within a vertical well or laying the coiled tubing horizontally on a straight stretch of land or road, and then progressively feeding the cable into the coiled tubing. As can be appreciated, this can result in a time-consuming, expensive and labor-intensive solution to the problem of installing cable within coiled tubing.
Today, three primary methods are employed to install cable into coiled tubing. The first method utilizes a simple flow tube attached directly to the coiled tubing, which creates a single pressure system in which the pressure drop across the flow tube combines with the pressure drop across the coiled tubing. Depending upon the stiffness of the cable selected, it may not be possible to reach the pump rates necessary to carry the cable before maximum pressure is exceeded.
The second method uses a mechanical injector to push the cable into the tubing. While this method provides a greater chance of success as compared to the first method, it is nonetheless quite inefficient. The injector is comprised of two polycarbonate wheels that grip the cable and thereby create a mechanical force independent of the pump rate in the coiled tubing. During the course of installation, the wheels typically wear out after approximately 8,000 feet of cable has been injected, and changing the wheels is very labor intensive and requires a considerable amount of down time. In addition, if there is fluid of any kind on the cable, or if the cable stops moving at any point, the wheels can lose traction and become unable to function and often must be changed. This method also tends to cause a significant amount of twisting and permanent deformation in the cable. As a result, cables installed using the mechanical injector cannot be removed and re-installed in a different string of coiled tubing, if desired.
The third method involves using a capstan device to deploy the cable. This method is expensive, labor intensive, and the capstan device is much too large for easy transport to well sites.